Work will continue along two lines, using developing tissues. 1) We have found that transport of alpha-aminoisobutyric acid (AIB) into rat brain slices is stimulated by thyroxine in vivo, with a maximum effect at about 14 days of age. Work will be undertaken to define more specifically the transport system (s) altered by the hormone. Studies will include further definition of the transport system(s) used by AIB in brain with and without thyroxine treatment; and effects of thyroxine on transport of amino acids that model for systems not used by AIB, to determine which systems are altered by thyroxine. 2) We have found that insulin in vitro does not stimulate AIB transport into diaphragms from 1-day old rats, but gives increasing stimulation as donor rats, ages up to 5 weeks. The lack of response in 1-day tissues is related to a naturally high plasma insulin level at that age. These high plasma levels do not cause elevated AIB transport in vitro, but do block the ability of insulin in vitro to produce stimulation. Studies will be aimed at determining the reason for this block, and will include measurements of insulin binding sites on diaphragms under various conditions.